Crossword Genius

Housman initially working for donkey's years (4)

Ross

I believe the answer is:

aeon

I'm a little stuck... Click here to teach me more about this clue!

'years' is the definition.
(aeon is a kind of years)

'housman initially working for donkey's' is the wordplay.
I cannot really understand how this works, but
'working' could be 'on' (e.g. a machine is working or on) and 'on' is present in the answer.
The remaining letters 'ae' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for aeon that I've seen before include "Vast time period" , "Vast age" , "long term" , "A very long time, an age" , "An age; period" .)

I've seen this clue in The Guardian.
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